Many modern industrial processes involve contacting liquids or solids with a gas which produces a suspension of liquid droplets and/or solid particles in the gas. Frequently, the suspended substance is a hazardous or expensive material or merely is nuisance contamination in a valuable gas stream. Filtration of droplets and particles from gas thus becomes important for many commercial applications such as recovery or containment of valuable or dangerous particulate materials; venting purified exhaust gas for disposal to the environment; and purifying a contaminated gas stream for use as a raw material in a later process step.
Membrane technology increasingly is applied to the filtration of industrial gases. Because fluoropolymers have certain physical properties such as good hydrophobicity, inertness to a variety of chemical and biological materials, and good thermal stability, these materials, and especially microporous, expanded polytetrafluoroethylene ("E-PTFE"), are popular for use in porous membrane filters. One notable shortcoming is that oil "wets" E-PTFE. Wetting refers to the affinity of a liquid for the membrane material. E-PTFE can become so wet with oil that the oil clogs the pores of the membrane. This reduces and sometimes totally blocks the gas flow through the membrane. Oil is present in a large number of gas processing applications, including oil lubricated compression and automotive applications, for example. Hence, the oleophilic nature of E-PTFE significantly reduces the effectiveness of this material in membrane filtration.
A gas permeable membrane with both high hydrophobicity and oleophobicity has been sought for improved gas filtration performance. U.S. Pat. No. 5,554,414 of Moya et al. provides a process for producing a composite porous article having a porous polymeric substrate and a hydrophobic/oleophobic polymeric surface formed from a cross-linked ethylenically unsaturated monomer containing a fluoroalkyl group. The polymeric surface is formed by coating a porous membrane substrate with a solution of a polymerizable monomer, a cross-linking agent, and a polymerization initiator. The polymerizable monomer is polymerized and cross-linked onto the porous membrane substrate in a way that the entire surface of the porous membrane, including the inner surfaces of the porous membrane, is modified with a cross-linked polymer. However, the composite porous article retains substantially all of the original properties of the substrate, particularly porosity.
U.S. Pat. No. 5,116,650 of Bowser describes the use of an amorphous copolymer of 10-40 mole percent tetrafluoroethylene ("TFE") and a complementary amount of perfluoro-2, 2-dimethyl-1, 3-dioxole ("PDD") for a gas filter. The amorphous copolymer is coated onto a gas permeable material which has passageways, or continuous pores, through the material. The amorphous copolymer coats at least a portion of the interior of the passageways but does not block them.
The above-cited references describe completely porous membrane structures for gas filtration. Gas molecules can travel readily through such a structure via the passageways formed by the pores. As a result porous structures generally provide high gas flux, that is, gas transmission per unit of filter surface area. Hence, a moderately sized porous filter element usually can transfer gas at industrially acceptable rate. Although the pores are coated with enhanced oleophobic compositions to reduce the tendency of oil to adhere to the membrane, the open pores still provide the opportunity for oil to penetrate and eventually clog the membrane. Solid particles that may be suspended in a gas can also enter and occlude the pores. Additionally, penetrating liquid and solid contaminants can become embedded in the pores and can be difficult to clean out. Thus, the gas flow through a porous membrane gas filter can decrease over time in service.
A practical, non-porous permeable membrane for a gas filter has not been available previously. Gas flux through a non-porous permeable membrane is directly proportional to permeability of the membrane composition and inversely proportional to membrane thickness. Most polymeric compositions have low gas permeability. Consequently, to provide a filter element of practical size surface area with industrially significant gas flux, a non-porous membrane of even moderately high permeability would need to be extremely thin. Heretofore a method for making a sufficiently thin non-porous gas permeable membrane for a gas filter has not been known in the art.
A process for making a membrane structure comprising an ultra-thin, continuous layer of a non-porous, gas permeable polymer composition now has been discovered. The polymer composition has good permeability which provides high initial gas flux. Additionally, the non-porous structure of the continuous layer imparts superior resistance to oil and solid particle penetration and improved stability of gas flux. Furthermore, if the novel membrane structure becomes fouled, it can be cleaned easily to restore gas flux to near-original gas transmission rate. As a consequence of this invention, it is now possible to produce a very thin film of a non-porous, gas permeable polymer in a membrane structure adaptable for use as a gas filter and for other gas transfer operations.
Accordingly, this invention provides a process for making a membrane structure comprising the steps of:
(a) dissolving a gas permeable polymer in a solvent to obtain a coating solution; PA1 (b) selecting a microporous substrate of a pore size effective for filtering dissolved polymer from the coating solution, the substrate having a first side, and a second side; PA1 (c) contacting the first side of the microporous substrate with the coating solution; PA1 (d) making the solvent flow through the microporous substrate to the second side; PA1 (e) removing coating solution and solvent from the membrane structure; and PA1 (f) evaporating solvent from the membrane structure, thereby forming a continuous, non-porous layer of the gas permeable polymer on the first side. PA1 (a) dissolving a gas permeable polymer in a solvent to obtain a coating solution; PA1 (b) providing a filter module including PA1 (c) causing the coating solution to flow through one of the shell side cavity and the tube side cavity; PA1 (d) making the solvent flow from the coating solution through the microporous hollow fibers to the other of the shell side cavity and the tube side cavity; PA1 (e) draining coating solution and solvent from the module; and PA1 (f) evaporating solvent from the hollow fibers thereby forming a continuous, non-porous layer of the gas permeable polymer on one side of the hollow fibers. PA1 a microporous substrate; and PA1 a non-porous gas permeable layer on the substrate and continuous over the entire filter surface area of an amorphous copolymer of perfluoro-2,2-dimethyl-1,3-dioxole having a permeability to oxygen of at least 100 barrers at a temperature below the glass transition temperature of the amorphous copolymer. PA1 a microporous substrate having a pore size of about 0.005-0.1 .mu.m; and PA1 a non-porous gas permeable layer on the substrate and continuous over the entire filter surface of an amorphous copolymer of perfluoro-2,2-dimethyl-1,3-dioxole having a permeability to oxygen of at least 100 barrers at a temperature below the glass transition temperature of the amorphous copolymer.
In another aspect, the present invention also provides a process for coating hollow fibers with an ultra-thin, continuous layer of a gas permeable polymer.
Additionally, there is provided a process for making a gas filter comprising the steps of:
(1) an elongated casing having two ends, the casing defining a shell side cavity; PA2 (2) a first tube sheet at one end of the casing having a first tube sheet outboard face; PA2 (3) a second tube sheet at the other end of the casing having a second tube sheet outboard face; PA2 (4) a plurality of open ended, microporous hollow fibers extending in substantially parallel alignment within the casing from the first tube sheet outboard face to the second tube sheet outboard face, the hollow fibers collectively defining a tube side cavity; wherein the pore size of the hollow fibers is effective to filter dissolved polymer from the coating solution; and PA2 (5) at least one shell side port through the casing;
In another aspect, the present invention provides a method of separating a gas from an aerosol comprising permeating the gas through a filter surface area of a membrane structure including
Still further there is provided a novel gas filter comprising a membrane structure having a filter surface area for permeating a gas to separate suspended droplets from the gas, the membrane structure comprising: